In most students' introduction to rigorous proof-based mathematics, many of the initial exercises and theorems are just a test of a student's understanding of how to work with the axioms and unpack various definitions, i.e. they don't really say anything interesting about mathematics "in the wild." In various subjects, what would you consider to be the first theorem (say, in the usual presentation in a standard undergraduate textbook) with actual content?

Some possible examples are below. Feel free to either add them or disagree, but as usual, keep your answers to one suggestion per post.

Number theory: the existence of primitive roots.

Set theory: the Cantor-Bernstein-Schroeder theorem.

Group theory: the Sylow theorems.

Real analysis: the Heine-Borel theorem.

Topology: Urysohn's lemma.

Edit: I seem to have accidentally created the tag "soft-questions." Can we delete tags?

Edit #2: In a comment, ilya asked "You want the first result after all the basic tools have been introduced?" That's more or less my question. I guess part of what I'm looking for is the first result that justifies the introduction of all the basic tools in the first place.

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3

This question seems to have long outgrown its usefulness (and some of the more recent additions have been, IMHO, lousy). Voting to close.
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Todd Trimble♦Mar 1 '12 at 16:03

I was about to post that. I should make the addendum though that while Lagrange stated his theorem in 1770, the first full proof occurred 30 years later at the same time as the first proof of the insolubility of the quintic.
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Jason DyerOct 24 '09 at 20:42

2

LaGrange's theorem is just an application of the definition of an equivalence class.
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Harry GindiDec 13 '09 at 15:26

25

Fermat's last theorem is just an application of the definition of a natural number.
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Steven GubkinMar 5 '10 at 0:15

4

@Marcos : "I don't understand why he didn't prove it generally". Presumably because the definition of a group was only given in the mid-19th century.
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Laurent BergerMar 1 '12 at 14:03

Bezout's Theorem is a nice theorem but it is hardly surprising in its proper setting (algebraically closed field, taking into account multiplicities and points at infinity).
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lhfOct 30 '09 at 9:18

6

But it is a great motivator for schemes and cohomology: To put projective and affine space in the same framework, you need gluing. To get the right formulas for higher order contact, you need the scheme theoretic intersection of curves. When you approach the theorem cohomologically, it reduces to just intersecting lines (which is a conceptually beautiful way to approach the proof). So not only is it simple to understand, it can be used as motivation for very deep ideas.
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Steven GubkinNov 12 '09 at 20:04

Quadratic reciprocity feels to me like a "first nontrivial statement" without an obvious branch of mathematics. Not number theory -- there are things like the infinitude of the primes, and "algebraic number theory" doesn't seem quite right either...

I'm not sure I like this one - although it can be hard to prove when you don't have much machinery, it's basically a tautology deriving from the definition of the fundamental group and not a very insightful result. I'd say the first nontrivial theorem of algebraic topology (at least on the homology side of things) is Poincaré duality - after all that was one of the things that launched the subject. Although there are easier results (Brouwer fixed point), they do not really pertain to algebraic topology itself. Considering homotopy, there should be something about homotopy groups of spheres.
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Sam DerbyshireOct 25 '09 at 2:47

2

This is an important first result because now we can prove the Brouwer fixed point theorem (usually the fundamental group comes before homology right? the other things you need for that result are much more straightforward).
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Sean TilsonMar 5 '10 at 21:05

3

This is the most important computation in algebraic topology, in my opinion. Everything else ultimately derives from it.
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Jeff StromJul 23 '10 at 18:06

I thought about that, but I think category theorists would consider the Yoneda lemma trivial. Not to say that it's easy to understand, but it does follow directly from the category axioms.
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Qiaochu YuanOct 24 '09 at 20:36

21

Though Yoneda's lemma isn't non-trivial, I feel like understanding its significance is definitely a non-trivial step. Schur's lemma in representation theory and Nakayama's lemma in algebra have a similar feel to them. They're pretty trivial to prove, but can take a while to really grok them.
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Anton GeraschenkoOct 24 '09 at 20:56

11

Maybe it's more correct to say that Yoneda is the last trivial theorem?
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Harrison BrownOct 25 '09 at 5:52

Number theory: Different undergraduate textbooks approach the subject differently, of course. But the irrationality of the square root of 2 and the infinitude of primes are contentful theorems that are certainly very early historically, and also very early in at least some textbook treatments of the subject.

I have to admit when I say "number theory" I always think "in the sense of Gauss." The infinitude of the primes is a better candidate now that I think about it.
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Qiaochu YuanOct 24 '09 at 20:39

4

Perhaps you could even argue that the irrationality of the square root of 2 and the infinitude of primes are the first nontrivial theorems in mathematics as a whole. The Pythagorean theorem seems to be another candidate in this direction.
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Michael LugoOct 24 '09 at 21:59

3

Probably Pythagorean before "sqrt(2) is irrational". Without a^2 + b^2 = c^2 with a = b = 1, we have no reason to consider sqrt(2) in the first place.
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Chad GroftFeb 22 '10 at 17:31

Although of course, this is just a categorification of the corresponding statement for finite sets...
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Scott Morrison♦Oct 24 '09 at 21:42

3

You could say that the rank nullity theorem is a direct consequence of the fact that all modules over fields are free (and hence projective). This may be a warped way of looking at things but I have to admit, thats how I remember it now :)
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Grétar AmazeenOct 25 '09 at 2:08

3

I think that various theorems about bases (every two bases have the same cardinality, every linearly independent set can be extended to a basis, and so on) are nontrivial and certainly come earlier. Why non-trivial? Because they may fail for modules over other rings, even if the module is free (but the ring may be noncommutative) or for f.g. modules over commutative rings (maximal linearly independent systems may have different cardinality, mathoverflow.net/questions/30066/…)
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Victor ProtsakJul 24 '10 at 5:33

Although the Gauss-Bonnet theorem was cited for differential geometry of surfaces, I really think that the first striking result in this subject is Gauss's Theorema Egregium, which is not obvious from the definition of Gaussian curvature (which makes explicit reference to the ambient space). But the Gauss-Bonnet theorem is certainly the first really deep theorem one encounters in differential geometry.

I would think of Completeness or Compactness of first order logic as a more reasonable answer. Incompleteness is interesting, but it feels more like a side branch of the theorem tree than the main trunk (whatever that means).
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Richard DoreOct 25 '09 at 6:40